Piezoelectric micromotors of different shapes and characteristics for driving moveable components of machines are well known in the art. U.S. Pat. No. 5,616,980, which is incorporated herein by reference, describes a piezoelectric micromotor capable of providing a relatively large amount of power for driving a moveable body.
The piezoelectric micromotor comprises a thin rectangular piezoelectric plate having short and long edge surfaces and large parallel face surfaces. One of the large face surfaces has a single large electrode covering substantially all of the area of the face surface. The other large face surface has four “quadrant” electrodes arranged in a checkerboard pattern in which each of the quadrant electrodes covers substantially all of a quadrant of the face surface.
To transmit motion to a moveable body, a region, hereinafter referred to as a “contact region”, of a short edge surface of the piezoelectric plate, or a wear resistant extension thereof, is resiliently pressed to a surface area of the body. Voltage differences are applied between quadrant electrodes and the single large electrode, to generate elliptical vibrations in the plate and contact region. Motion is transmitted to the body in directions parallel to the short edges from the contact region of the plate. Typically, to move the body in one direction along the short edge, a same voltage is applied to each quadrant electrode of a first pair of diagonally situated quadrant electrodes. Quadrant electrodes in the second pair of “diagonal” electrodes are floating or grounded. To reverse direction of motion, voltage is applied to the second pair of quadrant electrodes and the first pair is floating or grounded.
Typical operating voltages for the quadrant electrodes range from 30–500 volts and depend upon the geometry of the piezoelectric plate, the mass of the body and a desired speed with which the body is to be moved. Speeds of from 15–350 mm/sec for moveable bodies are reported in the patent for a micromotor having dimensions of 30 mm×7.5 mm×2–5 mm thick. Higher speeds are possible for micromotors of this construction having different dimensions.
In many instances, a piezoelectric micromotor that can provide relatively large amounts of power for driving a moveable body is required to operate as a component in a battery driven device. These devices are generally low voltage devices and it would be advantageous to have piezoelectric micromotors that can provide the power and traveling speeds provided by the piezoelectric micromotor described in the above referenced US patent at lower operating voltages.
Low voltage piezoelectric micromotors are generally formed from a stack of layers of piezoelectric material. Many of these types of “multilayer” micromotors cannot deliver the power and provide the speeds of the described piezoelectric micromotor. In addition, during operation, shear forces often develop in the multilayer micromotor that stress bonds that hold layers of the micromotor together. This occurs especially when the multilayer micromotor accelerates or decelerates a moveable body to which it is coupled. The shear forces tend to damage the bonds and often result in accelerated deterioration of the structure of the micromotor.
A low voltage piezoelectric multilayer micromotor that operates at driving voltages between 3–10 volts is described in an article entitled Multilayer Piezoelectric Motor Using the First Longitudinal and the Second Bending Vibrations, by H. Saigoh in Jpn J. Appl. Phys, Vol. 34 (1995) Pt 1, No. 5B pp. 2760–2764. The multilayer micromotor is shaped as a long parallelepiped formed from a stack of 40 thin rectangular layers of piezoelectric material. Voltage differences are applied to electrodes located between the layers to excite elliptical vibration modes in the micromotor. To transmit energy from the vibrations to move a body, the micromotor is pressed to the body so that two contact regions on one of the outer layers in the stack are pressed to the body. Motion is transmitted in either of two directions parallel to the long dimension of the micromotor. The micromotor is reported to move a slider weighing 49 grams at speeds up to 200 mm/s.
U.S. Pat. No. 5,345,137 to Funakubo et al describes multilayer piezoelectric micromotors that comprise a tall stack of thin piezoelectric plates. The height of the stack is substantially greater than its dimensions perpendicular to the height. A contact region for coupling the micromotor to a moveable body is located on a face surface of a top plate in the stack. Motion is transmitted to a moveable element in directions perpendicular to the height. Whereas this type of multilayer micromotor can provide relatively large amounts of power at high speeds, substantial shear forces stress bonds that join plates in the stack.
Because a piezoelectric micromotor is generally required to operate with fast response times and accurately control motion of a body it moves, it is positioned so that a contact region of the micromotor is always pressed to the body. As a result, it is started “under load”, i.e. while it is coupled to the body it moves. Therefore, to start the micromotor and initiate motion in the body a large “overshoot starting-voltage” must generally be applied to the micromotor. After motion starts, the applied voltage must be quickly reduced to a substantially lower voltage to maintain a desired speed for the body. The body therefore often begins to move with an uncontrollable lurching. In addition, jitter of as much as a few milliseconds is common in the time at which the body begins to move after the micromotor is turned on.
A problem also sometimes exists when a piezoelectric motor is required to move a body from a first location and position it accurately at a second location. It is generally difficult to control deceleration of the body as it approaches the second location with sufficient accuracy to bring the body directly to the second location so that it is accurately positioned at the second location. Often the body overshoots or undershoots the second location. To accurately position the body, the micromotor is usually operated to “jockey” the body back and forth about the second location until the body is positioned at the second location with a satisfactory degree of accuracy. The jockeying occurs during a period of time, known as a “settling time” that often lasts for as long as several milliseconds to several tens of milliseconds.
It would be desirable to have a high power, low voltage piezoelectric micromotor that provides improved control of motion that it imparts to a body it moves during “start up” and positioning of the body.